The sampling protocol is the procedure used to select units from the study population to be measured. The goal of the sampling protocol is to select units that are representative of the study population with respect to the attribute(s) of interest. The sampling protocol deals with how and when the units are selected and how many units are selected.
Michelson decided to sample a number of units one hour after sunrise and one hour before sunset for a number of days between June 13 and July 2. The units were selected in groups of 10 with from one to six groups taken per time period. Units were selected by Michelson and, on two occasions, by his assistants Lieutenant Nazro and Mr. Clason. In all, 1000 units were sampled. Over the course of the sampling, other explanatory variates were manipulated (speed of rotation of the mirror, the angle of inclination of the rotating mirror etc.) Michelson recognized the importance of selecting units with different values for these explanatory variates so that he could verify that they did not affect the measured velocity of light. Consider, for example, his discussion of observer bias in the final section of the paper. To deal with this issue, additional sets of measurements were taken by another observer who was blind to Michelson's results. There was no systematic difference in the two sets of values.
We call sample error the difference between the attribute of interest in the study population and the corresponding attribute in the sample. As with measuring processes, there may be bias and variability associated with the sampling protocol. These are properties of the protocol and not of any particular sample of units. As with the measuring process, sampling bias and sampling variability are defined in terms of the properties of the sample error when repeatedly applying the sampling protocol to the study population. These replications are always hypothetical which means that we can describe sampling bias and variability only through a model of the sampling protocol. We postpone discussion of this model to the Analysis section although in the active use of PPDAC, mathematical models for the potential sampling protocol (and measuring processes) are used to help with issues such as sample size determination.